This invention relates to a method to determine the correct reflectance values of translucent objects at one or more spectral wavelengths and one or more illumination and viewing geometries. In particular, the method corrects for errors caused by lateral diffusion of instrument illumination light, within the object, to areas that cannot be viewed by the instrument detection system.
The reflectance of an object is measured by illuminating some portion of the object with light from one or more directions and measuring the magnitude of the light reflected from the object in one or more directions. The light from the illuminating source may or may not passed through a monochromator or one or more spectral filters prior to illuminating the object. The light reflected by the object may or may not be passed through a monochromator or one or more filters prior to be evaluated by one or more detectors. If filters with one or more specific band passes are used and the instrument is calibrated with a reflectance standard, the instrument might be referred to as a filter reflectance densitometer or a filter calorimeter. If a monochromator is used and measurements are made at several wavelengths, the instrument would generally be referred to as a spectrometer. If the output of the spectrometer is referenced to a previously or concurrently measured calibration standard, the instrument is usually called a spectrophotometer. If a number of spectral reflectance measurements at specified wavelengths are made with the spectrophotometer and are processed a manner set forth by the CIE or ASTM (xe2x80x9cStandard Practice for Obtaining Spectrophotometric Data for Object-Color Evaluation,xe2x80x9d ASTM Designation E 1164(83), American Society for Testing Materials, Philadelphia, Pa., incorporated herein by reference), the instrument and processor represents a spectrocolorimeter.
When the reflectance of a translucent object or material is measured, some of illuminating light laterally diffuses within the body of illuminated object to locations beyond the edge of the illuminating area. The laterally diffused light forms a dim halo around the illuminated area, and the intensity of the light near the edge of the illuminated area is reduced relative to that in the center of the area. If a detector views only the illuminated area, the measured reflectance value will be in error because the detector has not viewed the laterally diffused light.
Hsia, NBS Technical Note 594-12 (1976), has referred to this measurement error as translucent blurring error. Atkins and Billmeyer, Materials Res. and Std., 6 (1966), pp 564-569, refer to it as edge-loss error. Hunter and Harold, The Measurement of Appearance, 2nd ed, Wiley, N.Y., p. 410 (1987), call it translucency error. Spooner, Proc. of SPE-RETEC CAD, Charleston, S.C., Sept. 25-26, 1995, uses the term lateral diffusion error (LDE) to describe the process. All of these references are incorporated herein by reference.
Various methods have been developed to correct for this error. In xe2x80x9cover-viewingxe2x80x9d the area viewed by the detector is increased so that all of the light reflected by the sample, even the light which is laterally diffused out of the illuminated area, can be seen by the detector. ISO Ref. No. 5/4 1983 (E), International Organization for Standardization (xe2x80x9cISO 5/4xe2x80x9d), incorporated herein by reference, which specifies geometry and optical parameters for measurement of reflectance density of photographic products, specifies that the boundary of the viewed area should be at least 2 mm beyond the edge of the illuminated area. In xe2x80x9cover-illuminationxe2x80x9d the illuminated area is larger than the area viewed by the detector. If the conditions set forth in Clarke and Perry, xe2x80x9cHelmholtz Reciprocity: its validity and application to reflectometry,xe2x80x9d Lighting Res. and Tech., 17 (1985), pp 1-11, are met, then the error reduction achieved will be the same whether the illumination or viewing area is increased by a similar amount. Spooner, SPE Color and Appearance Division RETEC Proc., Oct. 1-2, 1996, St. Louis, Mo., discloses a method for deriving a LDE correction which used measurements made at two positions relative to the instrument port.
When over-illumination, over-viewing, or two measurement position method is used to minimize LDE, it is assumed that the sample has a uniform surface and bulk content within all areas illuminated and viewed. The extent of over-illumination or over-viewing is highly dependent on the translucency of the sample. For instance, some translucent plastic samples require illumination/viewing aperture differences of 20 mm or more. According to ISO 5/4, only a 2 mm on the side difference is used to obtain a useful measurement of photo papers. An instrument designed for plastics measurements would minimize LDE when used for measuring photo papers. However, its aperture sizes might be impractical for the measurement of commonly used density wedges.
The methods for reducing LDE involve measuring the desired sample area and some area adjacent to the desired object area. In the printing industry, it is common practice to use color print control strips with 5 mm square elements. If the relative aperture size criteria of ISO 5/4 are used, then measuring a print control element could be accomplished by illuminating a 1 mm area in the center of the element and viewing a 5 mm area. However, if the element is printed using a halftone screen, this geometry may give a measured value that is dependent on the positioning of the object relative to the instrument. If the viewed area was increased to 5 mm and the illuminated area was increased to 9 mm, then illuminated areas of different color adjacent to the desired object area would affect the measurement. A reflectance measuring system that could illuminate the entire 5 mm square area and take measurements from the entire 5 mm area with little or no LDE uncertainty in the measured values would be highly desirable.
Thus, a need exists for a method that can give corrected reflectance values for translucent objects without the use of over-illumination-or over-viewing so that only a small area of the object is examined. Such a method should be able to provide corrected measurements for objects with a wide range of translucencies without requiring mechanical changes in aperture sizes.
The invention is a method that determines corrected reflectance values for translucent objects without the use of over-illumination or over-viewing and without requiring mechanical changes in aperture sizes once a calibration curve has been determined. This method derives a LDE corrected measurement value by viewing essentially only the area of the object lighted by the instrument illumination system.
The method comprises the steps of:
a) determining a uncorrected reflectance for a measured area of the object by:
i) illuminating an area of the translucent object to produce the measured area,
ii) measuring the spatial distribution of the light reflected by the measured area, and
iii) calculating the uncorrected reflectance for the measured area;
b) calculating at least one SDV for the object:
c) determining a normalized LDE for the object by comparing the at least one SDV for the object with at least one predetermined relationship between SDV and normalized LDE, in which the number of SDVs for the object and the number of relationships is the same;
d) determining an LDE for the object from the normalized LDE;
e) calculating the corrected reflectance value for the object by adding the LDE for the object to the uncorrected reflectance.
In another embodiment, the invention is an apparatus for determining the corrected reflectance of an object.